An internal combustion engine of the type mentioned in the introduction is used for example as a motor vehicle drive unit. Within the context of the present invention, the expression “internal combustion engine” encompasses Otto-cycle engines, diesel engines and also hybrid internal combustion engines, which utilize a hybrid combustion process, and also hybrid drives which comprise not only the internal combustion engine but also an electric machine which can be connected in terms of drive to the internal combustion engine and which receives power from the internal combustion engine or which, as a switchable auxiliary drive, additionally outputs power.
Supercharging of an internal combustion engine serves primarily for increasing power. The air required for the combustion process is compressed, as a result of which a greater air mass can be supplied to each cylinder per working cycle. In this way, the fuel mass and therefore the mean pressure can be increased.
Supercharging is a suitable means for increasing the power of an internal combustion engine while maintaining an unchanged swept volume, or for reducing the swept volume while maintaining the same power. In any case, supercharging leads to an increase in volumetric power output and a more expedient power-to-weight ratio. If the swept volume is reduced, it is possible to shift the load collective toward higher loads, at which the specific fuel consumption is lower. By means of supercharging in combination with a suitable transmission configuration, it is also possible to realize so-called downspeeding, with which it is likewise possible to achieve a lower specific fuel consumption.
Supercharging consequently assists in the constant efforts in the development of internal combustion engines to minimize fuel consumption, that is to say to improve the efficiency of the internal combustion engine.
For supercharging, use is generally made of an exhaust-gas turbocharger, in which a compressor and a turbine are arranged on the same shaft. The hot exhaust-gas flow is supplied to the turbine and expands in the turbine with a release of energy, as a result of which the shaft is set in rotation. The energy supplied by the exhaust-gas flow to the turbine and ultimately to the shaft is used for driving the compressor which is likewise arranged on the shaft. The compressor conveys and compresses the charge air fed to it, as a result of which supercharging of the cylinders is obtained. A charge-air cooling arrangement may additionally be provided, by means of which the compressed charge air is cooled before it enters the cylinders.
The advantage of an exhaust-gas turbocharger for example in comparison with a mechanical charger is that no mechanical connection for transmitting power exists or is required between the charger and internal combustion engine; such a mechanical connection takes up additional structural space in the engine bay and has a not inconsiderable influence on the arrangement of the assemblies. While a mechanical charger extracts the energy required for driving it entirely from the internal combustion engine, and thereby reduces the output power and consequently adversely affects the efficiency, the exhaust-gas turbocharger utilizes the exhaust-gas energy of the hot exhaust gases.
Problems are encountered in the configuration of the exhaust-gas turbocharging, wherein it is basically sought to obtain a noticeable performance increase in all engine speed ranges. In the case of internal combustion engines supercharged by way of an exhaust-gas turbocharger, a noticeable torque drop is observed when a certain engine speed is undershot. The effect is undesirable and is one of the most severe disadvantages of exhaust-gas turbocharging.
The torque drop is understandable if one takes into consideration that the charge pressure ratio is dependent on the turbine pressure ratio. If, for example, the engine speed is reduced, this leads to a smaller exhaust-gas flow and therefore to a lower turbine pressure ratio. This has the effect that, toward lower engine speeds, the charge pressure ratio likewise decreases, which equates to a torque drop.
In the prior art, it is sought, using a variety of measures, to improve the torque characteristic of an exhaust gas-turbocharged internal combustion engine.
One such measure, for example, is a small design of the turbine cross section and simultaneous provision of an exhaust-gas blow-off facility. Such a turbine is also referred to as a wastegate turbine. If the exhaust-gas mass flow exceeds a critical value, a part of the exhaust-gas flow is, within the course of a so-called exhaust-gas blow-off, conducted via a bypass line past the turbine. The approach has the disadvantage that the supercharging behavior is inadequate at relatively high engine speeds.
The torque characteristic of a supercharged internal combustion engine may furthermore be improved by means of multiple turbochargers arranged in parallel, that is to say by means of multiple turbines of relatively small turbine cross section arranged in parallel, turbines being activated successively with increasing exhaust-gas flow rate, similarly to sequential supercharging.
The torque characteristic may also be advantageously influenced by means of multiple exhaust-gas turbochargers connected in series. By connecting two exhaust-gas turbochargers in series, of which one exhaust-gas turbocharger serves as a high-pressure stage and one exhaust-gas turbocharger serves as a low-pressure stage, the compressor characteristic map can advantageously be expanded, specifically both in the direction of smaller compressor flows and also in the direction of larger compressor flows.
In particular, with the exhaust-gas turbocharger which serves as a high-pressure stage, it is possible for the surge limit to be shifted in the direction of smaller compressor flows, as a result of which high charge pressure ratios can be obtained even with small compressor flows and the torque characteristic in the lower engine speed range is considerably improved. This is achieved by designing the high-pressure turbine for small exhaust-gas flows and by providing a bypass line by means of which, with increasing exhaust-gas flow, an increasing amount of exhaust gas is conducted past the high-pressure turbine. For this purpose, the bypass line branches off from the exhaust-gas discharge system upstream of the high-pressure turbine and opens into the exhaust-gas discharge system again upstream of the low-pressure turbine. In the bypass line there is arranged a shut-off element for controlling the exhaust-gas flow conducted past the high-pressure turbine.
The internal combustion engine to which the present invention relates has at least one exhaust-gas turbocharger, the turbine, which comprises a turbine housing, of the at least one exhaust-gas turbocharger being a wastegate turbine. According to the prior art, the exhaust lines which adjoin the outlet openings of the at least two cylinders are at least partially integrated in the cylinder head and are merged to form a common overall exhaust line or merged in groups to form two or more overall exhaust lines. The merging of exhaust lines to form an overall exhaust line is referred to generally, and within the context of the present invention, as an exhaust manifold.
With regard to the configuration of the exhaust-gas turbocharging, it is sought to arrange the turbine or turbines as close as possible to the outlet of the internal combustion engine, that is to say close to the outlet openings of the cylinders, in order thereby to be able to make optimum use of the exhaust-gas enthalpy of the hot exhaust gases, which is determined significantly by the exhaust-gas pressure and the exhaust-gas temperature, and to ensure a fast response behavior of the turbocharger. A close-coupled arrangement not only shortens the path of the hot exhaust gases to the turbine but also reduces the volume of the exhaust-gas discharge system upstream of the turbine. The thermal inertia of the exhaust-gas discharge system likewise decreases, specifically owing to a reduction in the mass and length of the part of the exhaust-gas discharge system leading to the turbine.
For the reasons stated above, a turbine is generally arranged at the outlet side on the cylinder head, and the exhaust manifold is commonly integrated in the cylinder head. The integration of the exhaust manifold additionally permits dense packaging of the drive unit. Furthermore, the exhaust manifold can benefit from a liquid-type cooling arrangement that may be provided in the cylinder head, such that the manifold does not need to be manufactured from materials that can be subject to high thermal load, which are expensive.
The close-coupled arrangement of a turbine also enables the exhaust-gas after treatment arrangement to be arranged close to the outlet of the internal combustion engine. This offers advantages in particular after a cold start or in the warm-up phase of the internal combustion engine, because the path of the hot exhaust gases to the exhaust-gas after treatment systems is short or shortened. In this way, an exhaust-gas after treatment system reaches its light-off temperature or operating temperature more quickly after a cold start or in the warm-up phase.
The prior art also discloses other or additional measures for supplying exhaust gas at as high a temperature as possible to a turbine arranged in the exhaust-gas discharge system or to an exhaust-gas after treatment system provided in the exhaust-gas discharge system.
The exhaust-gas discharge system upstream of the turbine, that is to say an exhaust manifold or an overall exhaust line, may be equipped with thermal insulation, which counteracts cooling of the exhaust gas as it flows through the manifold or the overall exhaust line on the path to the turbine. The insulation serves as a barrier which impedes or hinders the extraction of heat from the exhaust gas.
A disadvantage of permanently acting thermal insulation is that the components provided in the exhaust-gas discharge system, and the exhaust-gas discharge system itself, are themselves subjected to high thermal load by the hot exhaust gas. This may lead to thermal overloading in particular at high loads and at full load.
To prevent thermal overloading of individual components of the internal combustion engine, according to the prior art, an enrichment (λ<1) is in some cases performed if high exhaust-gas temperatures are to be expected. Here, more fuel is injected than can actually be burned with the provided air quantity, with the excess fuel likewise being heated and evaporated, such that the temperature of the combustion gases falls. The approach is duly considered to be disadvantageous from energy-related aspects, in particular with regard to the fuel consumption of the internal combustion engine, and with regard to pollutant emissions, but is nevertheless recognized as being admissible and expedient for achieving the aim.
The exhaust-gas temperatures may basically also be reduced by means of a leaning (λ>1) of the fuel/air mixture. The effect is similar to that during an enrichment. Whereas too much fuel is injected during the enrichment, it is the case during a leaning that less fuel is injected than could be burned with the provided air quantity, that is to say more air is provided than is required for the combustion of the fuel, wherein the excess air participates in the combustion process, that is to say is jointly heated. The temperature of the combustion gases is reduced in this way. The temperature reduction as a result of leaning is considerably less pronounced than that during an enrichment because, by contrast to the excess fuel, the excess air does not need to be evaporated.
Against this background, it is an object of the present invention to provide a supercharged internal combustion engine, by means of which the disadvantages known from the prior art are eliminated and the operating behavior of which is improved.
The inventors herein have recognized the above-mentioned issues and have developed a supercharged internal combustion engine having at least two cylinders, having an intake system for the supply of charge air, and having an exhaust-gas discharge system for the discharge of exhaust gas and having at least one exhaust-gas turbocharger which comprises a turbine arranged in the exhaust-gas discharge system and a compressor arranged in the intake system, in which each cylinder of the internal combustion engine has at least one outlet opening for the discharge of the exhaust gases via the exhaust-gas discharge system, and each outlet opening is adjoined by an exhaust line, the exhaust lines of at least two cylinders merging to form an overall exhaust line, thus forming an exhaust manifold, and the turbine, which comprises a turbine housing, of the at least one exhaust-gas turbocharger is a wastegate turbine, and which internal combustion engine is distinguished by the fact that the exhaust-gas discharge system is equipped at least in regions with at least one cavity which can be used as thermal insulation, the at least one cavity forming a bypass which is at least connectable to the exhaust-gas discharge system upstream and downstream of the wastegate turbine and which is equipped with at least one shut-off element (e.g., a bypass valve).
By selectively flowing exhaust gas through a cavity in an exhaust manifold, it may be possible to provide the technical result of reducing engine emissions during engine starting and bypassing a turbocharger turbine at higher engine temperatures to reduce temperature increases of engine components. For example, holding exhaust gases in an outer cavity of an exhaust manifold during engine starting may improve heat retention in an engine exhaust system so that emissions devices in the exhaust system reach operating temperature sooner. Further, by allowing exhaust gases to enter the cavity and flow through the cavity, exhaust energy may bypass the turbocharger to reduce turbine temperatures as compared to if all the engine's exhaust flowed through the turbine.
The present description may provide several advantages. Specifically, the system may reduce engine starting emissions. In addition, the system may reduce the possibility of degrading heat sensitive engine components. Further, the system may be operated according to vehicle conditions to improve system operation.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.